Final answer:
The best answer isn't explicitly listed, but key abiotic factors in freshwater ecosystems include salinity, temperature, and dissolved oxygen, which affect the survivability and distribution of organisms.
Step-by-step explanation:
The major abiotic factor in freshwater ecosystems is not explicitly among the options provided; however, factors like salinity, temperature, and dissolved oxygen are critical.
Freshwater ecosystems are greatly influenced by several abiotic factors, which include not only those mentioned in the possible answers but others that play a substantial role in the survival of aquatic organisms. Salinity is one such factor, as the organisms in freshwater systems are typically not tolerant of high salt concentrations and cannot survive if salinity increases significantly. Temperature also affects these environments because of its impact on dissolved oxygen levels, where colder water typically holds more oxygen than warmer water.
The speed of water flow also influences the amount of oxygen available in the water; swiftly moving streams often contain higher levels of oxygen due to greater aeration while stagnant waters may be depleted of it. Other essential abiotic factors include light availability, which is crucial for photosynthesis, and consequent food chain dynamics. The width of a stream or river (the channel) and the source of the water do affect the ecosystem as well, impacting factors such as current strength and nutrient levels, but these are not as broadly significant as those related to water chemistry and physical conditions like temperature and oxygen levels.